A plurality of different methods have been developed for cationizing starch solutions. Cationizing agents conventionally used are tertiary or quaternary nitrogen compounds additionally including such a reactive functional group that is capable of effectively reacting with OH− groups of the starch. This kind of substituent group may be, e.g., an epoxy or a chlorohydrin group. Besides the OH− groups, oxidized starch may also have carbonyl and carboxyl acting as reacting groups.
The cationizing agent most commonly used today is 2,3-epoxypropyltrimethylammonium chloride or, alternatively, a corresponding cationizing agent with a chlorohydrin functional nature. These compounds are characterized in that they can establish an ether bond with the OH− groups of starch. Thus, they react with starch so as to form a compound which is stable over a very wide pH range. They are particularly stable especially over the basic pH range. This property is advantageous during long-term storage, since high pH gives products an increases resistance to microbiological attacks.
Based on their preparation technology, the cationizing methods can be categorized in three major groups:
1. Wet Methods
In these methods, cationization is carried out in an aqueous medium in which starch all the time can be in a slurry form, thus giving rise to an synonym term of slurry cationization. Furthermore, the starch may be partially or entirely dissolved during cationization. The latter process is called gel cationization.
2. Dry Cationization
This method is characterized in that the starch remains all the time in powdered form. The solids content may rise very high (even higher than 85%), while the degree of substitution (DS) usually stays smaller than 0.3.
3. Solvent Cationization
Herein, an organic solvent such as ethanol is used as liquid medium instead of water. Dissolution of starch in the solvent medium is generally tried to avoid, whereby it is possible to get the cationic product in powdered form. Yet, dissolution of the starch in the medium is an alternative not entirely excluded.
Most of the industrial-scale cationization processes employed today are based on the above-mentioned wet or dry starch preparation methods. Of the wet methods, methods based on slurry cationization techniques are primarily used. Cationization methods using solvents such as ethanol as a medium entail high operating costs. Their process investment costs are inflated i.e. by regeneration of solvents as well as by elevated fire risk and occupational safety factors.
Of the wet methods, slurry cationization is generally preferred when the goal is set for a relatively low degree of substitution (DS<0.1) and when the cationic starch product thus obtained is desired to be processed into slurry or powdered bulk shipping form. The gel technique is chiefly used when a high degree of substitution (DS from 0.1 to 1.0) is desired and an elevated process temperature is used. In this case, the cationic product is always in a dissolved form.
In suspension and slurry cationization, the starch is slurried in water to obtain a suspension of about 40-43% solids into which the cationizing agent is added. Simultaneously, the pH is controlled by sodium hydroxide addition to about pH 11-12, while the temperature is kept at about 40-45° C. Under these conditions, cationization occurs in about 6-16 h. The starch remains in slurry form during the entire cationization process. This technique is a leading method in the preparation of cationic internal and surface size starches having a DS value smaller than 0.1, typically less than 0.05.
A characteristic property of wet methods is that when the degree of substitution (DS) is elevated substantially higher than 0.1, the starch granules start to fragment and the cationic starch produced begins to swell and partially dissolve into water acting as the process medium. This is disadvantageous when the cationized starch is desired to be separated by filtration as a dry powder. However, in most applications a DS value smaller than 0.1 is quite sufficient.
Yet, there are applications in which the starch is required to have a substantially stronger cationic character. Such applications are for instance the use of cationic starch as a fixative, a retention agent, a flocculent, a dewatering chemical, a dispersant, a neutral size promotor or the like. Hereby a degree of substitution in the range of 0.1-1.0 or even higher is required, whereby the cationization must be carried out using the gel cationization technique. In using this method an economical maximum is considered to be a degree of substitution close to one.
By using an organic solvent the solubility of starch into the intermediary phase can be reduced substantially or even entirely prevented. By the solvent cationization method it is possible to produce starches of a considerably high cation equivalent value in which the DS value may be close to one.
The solubility of starch in cationization is essentially affected by the cationization temperature, the type and amount of catalyst used as well as the desired degree of substitution (DS). Also the degree of fragmentation affects the solubility of starch. Highly oxidized starches tend to dissolve more readily. Sodium hydroxide or lime is conventionally used as catalyst. In principle any base will do, which is able to separate a proton from the starch.
By dry cationization it is possible directly to get powdered cationic starch, but by this method it is more difficult to achieve an equally high degree of substitution than by the two other methods. In practice, already a DS value higher than 0.3 causes problems.
In the preparation of aqueous solutions of starches of very high cation equivalent value, it is even advantageous that the starch dissolves during cationization. In this way, an entire starch granule, during its gradual fracture, will be cationized entirely and, in practice even fully homogeneously. Generally, the same does not apply to dry and solvent cationization. In this context, a very high degree of cationization means a DS value of 0.1-1.0, which is equivalent to a nitrogen content of 0.8-4.5% in using the above-mentioned chemicals.
It is well known that the higher a nitrogen content, i.e. a DS value is attempted, the harder it will be to reach. This means that the higher the aimed DS value is, the lower the yield will be. The reasons thereto are on one hand related to steric factors in the starch structure and on the other hand on hydrolysis of the cationizing agent in the influence of water, sodium hydroxide and heat, a reaction competing with the cationization reaction.
In the previously known cationization methods, the yield of the cationization reaction in suspension and slurry cationization is about 70% (with a DS value of 0.05 to 0.1), and in gel cationization about 90% with a DS value smaller than 0.3 and about 75% with a DS value higher than 0.7. In dry cationization the yields are higher than in the above mentioned methods, but it is believed that the method is suitable only for obtaining a DS value smaller than 0.3. Likewise, slurry cationization is not known to be usable for a DS value higher than 0.1, principally due to filtration problems. The yield can be increased also by using a high-solids reaction environment. Decreasing the amount of water in the reaction mixture lowers the probability of the competing hydrolysis reaction. Previously this strategy has been applied to gel cationization as is described, e.g., in FI Patent No. 94135 and publication WO 95/18157.
Also continuously operating gel cationization methods (JP 7-68281 and JP 64-6001) are known, by which cationic starch solutions with a DS value smaller than 0.1 can be produced. In these methods the yields have been below 70% with a DS value smaller than 0.1.